Confused by Triple Glazing Justification

[Jilly]

Probably! I'm sad about it as I have a source of free wood and am in touch with my inner self sufficient hippy. 

[SteamyTea]

18 minutes ago, Jilly said:

am in touch with my inner self sufficient hippy. 

I heard a saying the other day, "if someone is wearing ethnic clothes, don't bother to argue with them".

[MortarThePoint]

It's not a utilitarian argument though. The real justification for WBS and open fires is emotional/aesthetic. People want a fire. I can see they're bad in cities but in the countryside not so much.

 

We're all wasteful even when being environmentally conscious. I doubt many here have built the smallest house they can bear. We make compromises and balances to fit our own priorities and that's not one size fits all.

[Ian]

On 14/04/2021 at 12:14, MortarThePoint said:

I'm confused by the motives of going for triple glazing as I like to do the maths behind these sorts of things. I see two primary reasons economic, reduced energy bills, and environmental, reduced 'carbon footprint'. I am pro green tech so start from a bias of wanting to include them.

 

Improved U-value: The U-value of a 1m2 window might typically improve from 1.2 W/m2K for double glazing to 0.8 W/m2K and so an improvement of 0.4 W/m2K. Based on heating the house for the coldest 6 months of the year (October to April inclusive):

Difference in U-values, dU = 0.4 W/m2K

average temperature difference across window, dT = 13 K (around my area for these 6 months, 19C inside, 6C outside)

Duration, t = 183*24 = 4392 hours

Window Area, A = 1 m2

Average heat flow, Q = A.dU.dT = 1*0.4*13 = 5.2W

Annual thermal energy, E = Q.t = 4392*5.2 = 22.8kWh per year

[Ignored: A double glazed window lets in more light (8% nominally) so there will be more solar gain in those months. That might work out as (guess) 50W/m2 and so an 4W difference between the two windows, but only for a fraction of the time so may amount to 1W average].

 

What am I missing here? Are the perceived benefits different?

 

 

Maths is not my strong point but I'd have thought that the difference between 1.2 W/m2K for double glazing and 0.8 W/m2K for triple glazing is a huge difference. Given how little extra £ that 3G costs I'd have thought it was a no-brainer. 

 

For the sake of argument, if we take an average house with a total wall area of 100m2 of which 20m2 is window then:

wall of 0.1W/m2K x 80m2 +  2G windows at 1.2W x 20m2 is an average U value over the wall of 0.32 W/m2K

It's the equivalent of building your wall to basic building regs compliance at 0.2W/m2K but installing 3G windows with a U value of 0.8 W/m2K. The U value is the same at 0.32 W/m2K

[MortarThePoint]

23 minutes ago, Ian said:

Maths is not my strong point but I'd have thought that the difference between 1.2 W/m2K for double glazing and 0.8 W/m2K for triple glazing is a huge difference. Given how little extra £ that 3G costs I'd have thought it was a no-brainer. 

 

For the sake of argument, if we take an average house with a total wall area of 100m2 of which 20m2 is window then:

wall of 0.1W/m2K x 80m2 +  2G windows at 1.2W x 20m2 is an average U value over the wall of 0.32 W/m2K

It's the equivalent of building your wall to basic building regs compliance at 0.2W/m2K but installing 3G windows with a U value of 0.8 W/m2K. The U value is the same at 0.32 W/m2K

 

Your numbers look correct. 100m2 with an average U-value of 0.32W/m2K and temperature difference of 13C will pass 100 * 0.32 * 13 = 416W of heat. An ASHP will consume about 140W of electricity to create that heat (based on COP=300%). 140W average for 6 months with an electricity price of 14p/kWh works out as 0.14kW * (24hrs * 183days) * £0.14/kWh = £86/yr. That's the total cost of all of the 100m2, not just the windows or the difference between the two window types.

 

My previous calculation identified 22.8kWh per year of heat for 1m2 as the difference between DG and TG. Based on 20m2 of windows, an ASHP (COP=300%) and 14p/kWh that would yield a total electricity saving of ((22.8kWh/m2 * 20m2) / 300%) = 152kWh per year, which amounts to 152kWh * £0.14/kWh = £21.28 per year.

 

I worked out on another thread that 1kWpk of solar panel sited well in my area would generate 1000kWh per year. So if I wanted to save that 152kWh, I would need 152kWpk plus whatever inefficiencies, call it 200kWpk needed. Solar costs around £1/Whpk so that's £200. A 350Wpk solar panel is about £350. If you have a solar array you would get more environmental benefit by adding a single panel to your array than by upgrading >30m2 of windows to triple glazing (in the South of England).

 

There are exceptions to this logic though. For example if you don't have an ASHP and want to keep all heat leaks down in order to avoid one. You may live somewhere that is much colder and gets less sun.

 

The perversion of all of this is that I may end up going TG due to the 'brochure value' of it. We're building for ourselves, but I have to be mindful of the impact of decisions. That said, I'm still not convinced we will go TG.

[SteamyTea]

2 hours ago, MortarThePoint said:

There are exceptions to this logic though

There are.

Generally the greater saving from triple glazing during the winter and at night, the two times that the PV is either working at a low level or not at all.

With PV it comes down to how much you can actually use, so the calculations go a bit wonky if you have to fit battery storage or a larger DHW cylinder.

 

It is always better to reduce the need for energy, rather than substitute those losses with energy.

4 hours ago, MortarThePoint said:

I can see they're bad in cities but in the countryside not so much.

The particulate problem is greatest where the WBS actually is, so rural people are hurting themselves at the same rate as city dwellers, but they just have a lower intake of particulates when they are outside.

Having said that, go to any village down here in the winter and you can smell the air. Not all rural people have no neighbours.

We are heading towards them being banned completely, just how long it takes.

[billt]

On 28/05/2021 at 08:32, Jilly said:

On the WBS discussion, has anyone researched masonry stoves? They seem to have a pretty clean burn.

 

Yes, if used as intended they are pretty clean for a wood burner. They are designed to burn DRY wood at very high temperatures and store the heat in the masonry. Of course, if you're using your wood burning stove correctly the fire temperature should be high as well which minimises pollution. The trouble is, on the evidence of my nose, people don't have a clue how to burn wood cleanly, even if they use properly dried wood.

 

Masonry stoves are bulky, so they waste space in your house and they are very expensive to construct, unless you build it yourself. They don't even have the excuse of aesthetic attraction as the fire is not visible.

 

We used to live in an isolated rural house with 10 acres of woodland so we used wood for heat. However, we used a gasifying log boiler which ran at high temperatures and produced little pollution - no visible smoke or perceptible smell once it was up to temperature, and no measurable increase in particulates.

 

We've moved to a more suburban area near a canal. Many of the surrounding houses have wood burners, despite also having mains gas central heating and the stink of partially burnt wood is evident most of the time during the heating season. Of course the canal boat residents contribute as well and seem to be very unaware of the pollution that they are creating.

[SteamyTea]

8 hours ago, billt said:

no measurable increase in particulates.

How did you measure them?

[billt]

A PurpleAir air quality sensor.

[SteamyTea]

1 hour ago, billt said:

A PurpleAir air quality sensor.

That uses the PMS1003 sensor I think.

I set one running in my spare room a while back, seemed to be greatly affected by temperature and humidity levels. I tried to calibrate it (found some info about calibration somewhere on the manufactures website), but struggled to get coherent readings.

It is one of those many things to do if I break a leg and have to stay at home for a few weeks.

[MortarThePoint]

On 14/04/2021 at 12:14, MortarThePoint said:

Environmental

A triple glazed unit contains many extras, but most obviously includes an extra sheet of glass.

Energy required to make glass, 21.9MJ/kg [1]

Density of glass, 2.5kg/(m2.mm) (2.5g/cm3)

Area of glass, 1m2 [overestimate as neglects frame width]

Thickness of glass, 4mm

Mass of glass 2.5*1*4 = 10kg

Entrained energy in glass, 21.9MJ/kg * 10kg = 219MJ = 60.8kWh

Energy debt payback time based on oil heating, (60.8kwh / 22.8kwh) = 2.7 years

Energy debt payback time based on ASHP(SCOP=3.5) heating, (60.8kwh / (22.8kwh/3.5)) = 9 years

There will be other uplifts in manufacturing 'carbon cost', but 9 years is a significant payback time and all that 'carbon' is upfront in manufacture and doesn't consider renewable sources of power generation that will come online over the next 9 years. Also, I would guess that glass manufacture is less likely to be using renewable sources of energy than the domestic electricity suppliers. I could believe that payback time heading beyond the lift of the unit.

 

What am I missing here? Are the perceived benefits different?

 

[1] https://www.sciencedirect.com/science/article/abs/pii/0166309781900614

 

This paper [2] suggests 3.08kg of CO2 per kg of flat glass. The UK electricity grid  generates at a CO2 per kWh of 0.23kg/kWh[3], that means that 1kg of flat glass is 13.4kWh or a 10kg glass pane is 134kWh. With an ASHP, just that part has a payback of over 20 years and without an ASHP and no PV it would be just under 6.

 

This sourcse [5] suggests 0.5kg of CO2 per kg of flat glass (620,139 tonnes CO2 / 1.3 million tonnes flat glass) but that's on the low side of sources.

This secondary source [4] puts the value at 8.4kg of CO2 per kg of glass but that seems high.

 

[2] https://www.researchgate.net/publication/288785551_CO2_emission_of_building_glass_production

[3] https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2020

[4] http://www.greenrationbook.org.uk/resources/footprints-glass/

[5] https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/416675/Glass_Report.pdf

[MortarThePoint]

From Pilkington themselves:

"The float process emits approx 0.8 kg CO₂ per kg glass sold from both direct (fossil fuels and carbonate decomposition) emissions and indirect (electricity generation) emissions. Approximately 0.3 kg CO₂ are released per kg glass sold during the extraction, processing and transportation of the raw materials to our sites. Therefore, approximately 1.1 kg CO₂ is released in order to manufacture 1.0 kg of non-processed, glass."

https://www.pilkington.com/en/global/commercial-applications/sustainability/sustainability-faq

 

That wording is a bit clumsy or obtuse. But if you take that to mean 1.1kg CO2 is released in order to manufacture 1.0kg of float glass stock, then that's more favourable and the numbers come out as (1.1CO2/kg*10kg) / (0.23CO2/kWh) = 48kWh to make a 10kg glass pane. With ASHP that's a 48/(22.8/3.5) = 7.3 year payback time. Over the next 10 years the grid is supposed to be getting greener though which would lower that 0.23 kg_CO2/kWh figure and raise the years to carbon payback. Processing and other aspects of making a triple glazed unit, let alone the frame uplift, will take than above 10 year.

 

With ASHP: 10 - 20 year payback CO2, 100year payback £.

Without ASHP: 3.5 - 6 year payback CO2, 30 year payback £.

[CalvinHobbes]

On 17/04/2021 at 09:30, Russell griffiths said:

I would say then that you are looking at the wrong manufacturers, my windows are have a 125mm wide frame, you won’t find any 2g windows with frames like that. 

 

We went 3g and at about 10%  uplift on 2g, they are silent and just feel completely different from any other 2g windows I’ve owned.  

Anyone any opinion or experience of this lot? 

https://www.profile22.co.uk/homeowner/